The invention relates to apparatus for the optical measurement of the concentrations in a substance to be analyzed, for example, oxygen in blood. The type of apparatus in question usually includes a housing containing a monochromator and a light-measuring unit, and an indicator chamber containing an indicator whose spectral response to the light from the monochromator is dependent upon variations in the concentration of the component of interest. The side of the indicator chamber facing the medium to be analyzed is closed off by a membrane which is permeable for the substance of interest. The side of the indicator chamber facing the monochromator is closed off by a layer of material through which the radiation of interest can pass (optode).
With measuring apparatuses of this type, a problem is posed by the fact that the substance to be analyzed may itself respond to the reaction to be measured with a spectral alteration of its own; i.e., the radiation to be measured, namely that attributable to the spectral dependence of the indicator upon the concentration of the component of interest, may have superimposed thereon so-called "background radiation" attributable to the spectral response of the substance to be analyzed itself. Additionally, there may occur at the boundaries of the interior space of the indicator chamber reflection effects which cannot be predicted and compensated for in advance; as a result, the functional relationship between the excitation radiation and the radiation to be measured is no longer determined exclusively by the concentration of the component of interest.